1. Field of Invention
This invention relates generally to tissue ablation and, in particular, to a lesion monitoring technique for controlling the removal of cardiac tissue to correct arrhythmias.
2. Description of the Related Art
Currently there are several medical and surgical treatments for abnormal rapid heart rhythms (arrhythmias). Medical treatments are usually attempted first with anti-arrhythmic drugs that control the arrhythmia by slowing cardiac conduction. When these treatments fail, surgical interventions become necessary.
One such form of intervention is cardiac ablation, a procedure that involves the removal of ventricular or atrial tissue to eliminate abnormal rapid heart rhythms by affecting cardiac conduction. Radio-frequency (RF) catheter ablation is a technique in which radiofrequency energy is delivered, via catheter, to a metallic electrode placed in contact with tissue within the heart. This energy causes tissue heating, which in turn produces the formation of a lesion. Radiofrequency catheter ablation has become a principal form of therapy for paroxysmal supraventricular tachycardia (rapid heart rhythm originating in the atria) and is also being used in an increasing number of patients for treatment of ventricular tachycardia (abnormal rapid rhythm originating in the ventricles) associated with coronary artery disease and other forms of heart disease. Ventricular tachycardia is a type of arrhythmia with high morbidity and mortality. RF current delivered through a standard 7F or 8F, 4 or 8 mm distal electrode has been highly successful for ablation of arrhythmogenic tissue that is critical to the initiation and maintenance of the arrhythmia. That is accomplished by placing the ablation electrode against the tissue, such as through accessory atrioventricular pathways (in patients with Wolff-Parkinson-White syndrome) and the atrial end of the slow atrioventricular-nodal pathway (in patients with atrioventricular nodal reentrant tachycardia). However, in approximately 1 to 10 percent of patients with accessory pathway and in 30 to 50 percent of patients with ventricular tachycardia associated with a healed myocardial infraction, the arrhythmogenic tissue may be located at the epicardial border zone and the RF energy delivered to the endocardium may not be sufficient to eliminate the arrhythmogenic tissue. In such cases, deeper lesions are necessary for a successful ablation procedure and adequate control of lesion formation becomes critical to avoid excessive tissue damage.
Under current practice, the catheter is guided through a vessel under fluoroscopy or equivalent technique to an appropriate site in the patient""s heart, where the electrode is positioned in the blood stream against the tissue to be ablated. It is important that contact between the electrode and the tissue be maximized in order to direct the RF energy toward the formation of tissue lesion rather than through the ambient blood phase. It is known that the impedance of the electrical system increases with greater contact of the electrode with the heart tissue. Therefore, this fact is used to ascertain when sufficient contact is established between the electrode and the heart tissue for carrying out the ablation procedure. A baseline impedance is measured when the electrode is known to reside entirely within the blood stream, and contact with tissue is assumed to have occurred when the impedance has increased by a predetermined amount set empirically for a given system.
It has been shown that tissue temperature must exceed 45-50xc2x0 C. for lethal tissue damage and lesion formation to occur, but the temperature cannot be so high (i.e., about 100xc2x0 C.) as to produce carbonization of the tissue and/or coagulation of the blood. For any given electrode size and tissue contact area, the RF-induced lesion size is a function of the RF power level and exposure time. Therefore, the objective of the procedure is to administer enough energy to heat the tissue to a temperature sufficiently high to induce a lesion and to achieve the desired degree of lesion formation, without also causing tissue carbonization or blood coagulation. Presently, the ablation procedure is carried out entirely on the basis of experience and empirical data. When the surgeon is satisfied that sufficient electrode/tissue contact exists based on a measurement of impedance, a certain amount of energy is delivered at a frequency within the RF range, typically at 250-500 kHz. The power input is maintained for a period of time known to be safe and reasonably effective to produce a useful depth of lesion. In some cases, a temperature sensor is coupled to the electrode in the catheter, so that the ambient temperature can be monitored to avoid an excessive rise and provide a useful parameter for monitoring progress during the procedure. Some systems include a feedback control system that adjusts RF power input to maintain the temperature constant after it has reached a predetermined set level considered optimal for ablation.
The problems with this prior-art methodology are that it relies almost entirely on educated guess work based on experience and past empirical data. If a temperature sensor is available, it provides a measure of the bulk temperature of the system, specifically at the location where the sensor lies, but it does not measure precisely the temperature of the tissue being treated, which is necessarily somewhat removed from the sensor""s location. Thus, excessive heating may be produced in the tissue even though it is not observed through the sensor. Another problem lies in the lack of ability to measure the depth of the lesion being produced during ablation. The power level and duration of RF energy delivery is set to produce a desired result based on experience, but the depth of the lesion actually produced cannot be monitored or controlled.
Therefore, there is a need for an ablation technique that includes a more precise indication of the temperature and depth of lesion of the tissue being treated. The present invention provides qualitative and quantitative measures of these parameters during the ablation procedure based on impedance measurements and on measurements of electrical parameters indicative of a change in the capacitance of the tissue.
The main objective of the invention is to provide a surgeon with an indication of the depth or volume of the tissue lesion produced by the administration of RF energy during the course of an ablation procedure, so that a progress can be monitored during the procedure and lesion formation can be optimized.
Another important objective is also to provide a measure of the temperature of the tissue being treated, so that the physician may benefit from current information regarding the onset of ablation and the potential for tissue carbonization and/or blood coagulation resulting from an excessive temperature rise as the procedure progresses.
Another important goal is a technique for estimating the tissue-to-blood ratio of interface with the ablation electrode, so that the physician may place the electrode in an optimal position against cardiac tissue.
Another goal of the invention is an approach that is suitable for implementing automatic control schemes based on electrical measurements that correlate quantitatively to tissue temperature and depth of lesion.
Still another goal of the invention is a method and apparatus that are suitable for implementation with existing instruments.
A final object is a procedure that can be implemented easily and economically according to the above stated criteria.
In accordance with these and other objectives, the invention consists of monitoring impedance and capacitance-related parameters in the electrical circuit of a tissue-ablation apparatus wherein RF electrical power is administered at predetermined frequencies. Tissue temperature has been found to correlate well with low-frequency impedance, or with the resistive component of impedance at any frequency. Therefore, one or both of these parameters are calculated and tracked during the ablation procedure to estimate tissue temperature. Similarly, tissue lesion formation has been found to correlate well with changes in the capacitive component of tissue impedance. Thus, this parameter can be used to track tissue lesion formation during the ablation procedure.
The invention further teaches to estimate the ratio of tissue-to-blood interface with the ablation electrode by measuring impedance at a very low frequency and a very high frequency. The difference between these two values divided by the low-frequency value is taken to be a measure of such ratio. Alternatively, other electrical parameters indicative of changes in the capacitive component of the system may be measured and the ratio is calculated as a function of these changes with respect to a baseline value.
Various other purposes and advantages of the invention will become clear from its description in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiments and particularly pointed out in the claims. However, such drawings and description disclose only some of the various ways in which the invention may be practiced.